1. Field of the Invention
This invention relates to a film key sheet which has a key top in a resin film and which is to be used as push-button switches of an apparatus endowed with key input function, such as an information terminal apparatus like a mobile phone and a PDA, a keyboard, and a controller for various types of apparatuses, and to a method of manufacturing the same.
2. Related Background Art
As an example of an apparatus endowed with key input function as mentioned above, FIG. 11 shows a mobile phone 1, which has a casing 2 containing a film key sheet 4 attached to a printed circuit board 3 (See FIG. 12). Attached to the back side of the film key sheet 4 is a rubber sheet 5 formed of silicone rubber, thermoplastic elastomer or the like. On the surface of the rubber sheet 5 opposed to the printed circuit board 3 are pushers 5a opposed to board contacts 3a having disc springs. And, to bring the pushers 5a into contact with the board contacts 3a to effect input operation on the mobile phone 1, key portions 7 and 8 exposed on the casing 2 are depressed.
There are fifteen key portions 7. As the cross sections thereof are shown in FIG. 12, formed in a hat-shaped sectional configuration on these key portions are key-shaped protrusions (key top protrusions) 9a through drawing of a resin film 9, comprising, for example, a transparent or translucent polycarbonate resin, in correspondence with the configuration of the key portions 7. On the portion of the back side of the resin film 9 where the fifteen key portions 7 are formed, colored layers 9b are formed by printing prior to the drawing, and the back side of the upper surface (the back surface of the upper surface) of the key-shaped protrusions 9a constitutes display portions 9c indicating characters, numerals, symbols, etc., which can be visually recognized from outside through the resin film 9. Then, inside the key-shaped protrusions 9a, there are formed key top main body portions 10 formed of polycarbonate resin or the like and having upper surfaces 10a and side surfaces 10b integrally secured to the inner surfaces of the key-shaped protrusions 9a (colored layers 9b). This construction of the key portion 7 is obtained by forming the key-shaped protrusions 9a through drawing of the resin key film 9 with the display portions 9c (colored layers 9b) formed beforehand by printing, and then performing injection molding on the key-shaped protrusions 9a with a molding resin forming the key top main body portions 10.
Regarding the three key portions 8, molding resin such as a transparent polycarbonate resin is injected inside the mold to predetermined positions of the resin film 9 and is allowed to cure, thereby providing a transparent, disc-like protruding key top 11. Also, on the back side of the portion of the resin film 9 where the key top 11 is provided, there are provided colored layers 9b formed by printing simultaneously with the colored layers 9b of the key portion 7. This portion provides display portions 9d having characters, numerals, symbols, etc., which can be visually recognized through the transparent key top 11 and the resin film under it. Mounted to the central one of the three key portions 8 is an annular directional key 13 secured to the rubber sheet 5 by adhesive 12.
Regarding apparatuses equipped with the film key sheet 4 as described above, there is an increasing demand for a reduction in thickness, and a further reduction in thickness is required of the conventional key sheet 4, too. As a method of achieving this objective, it might be possible, for example, to further reduce the height of the key top main body portions 10 of the key portion 7. It should be noted, however, that the height of the key top main body portions 10 depends on the foldability of the resin film 9 and the accuracy in the drawing by which the key-shaped protrusions 9a are formed by bending the resin film 9. With the current state of the art, there are limitations to a further reduction in height by this method.
Such limitations to a further reduction in height apply not only to the above-described film key sheet 4, shown as a conventional example, but also to all so-called film key sheets comprising resin films having a key top protruding therefrom, regardless of whether they are of the type which only have a key portion similar to the key portion 7 or of the type which only have a key portion similar to the key portion 8 and, further, regardless of the presence or position of a display portion.
Further, the following issue also deserves consideration. In the conventional film key sheet 4, in which the key portions 7 are formed by the resin film 9 and in which the key portions 8 are formed by the transparent key top 11, the two types of key portions can be distinguished from each other at a glance for the operator due to the differences in appearance in terms of textures and luster. Further, it is advantageously possible to achieve a multiple designs of the film key sheet 4 through the difference in appearance between the key portions 7 and 8. However, the conventional film key sheet 4 has a structural problem which makes it impossible to make a full use of this advantage. In the key top 11 of the key portions 8, the display portions 9d are formed on the back side of the resin film 9, so that visual recognition has to be effected not only through the key top 11 but also through the resin film 9, which means the clearness in the visual recognition of the display portions 9d is impaired by the presence of the resin film 9.
As a first method of solving this problem, it might be possible, for example, to form the display portions directly on the bottom surface of the key top 11 and then glue the key top to the resin film 9 by adhesive. This, however, would make it necessary to perform the positioning of the key top 11 one by one by hands to secure it at a predetermined position on the resin film 9, resulting in a very poor production efficiency. Further, in this case, the adhesive applied to the back surface of the key top 11 by means of a dispenser or the like, is normally put on a part of the back surface of the key top 11 so that it may not be squeezed out after adhesion. It is not a common practice to apply adhesive uniformly to the entire back surface of each key top 11. As a matter of fact, that measure would be very difficult to perform When illuminated from below, a film sheet obtained in that way would not provide uniform illumination property due to unevenness in the application of the adhesive. Thus, this method requires too much for the realization of a clear view of the display portions, which means it is far from being a drastic solution.
Further, as a second method of solving the above problem, it might be possible for example, as in the case of the other key portions 7, to print beforehand the display portions (colored layers) on the surface of the resin film 9 at positions corresponding to the key portions 8, and place the film in an injection mold, molding the key top 11 on the display portions (See JP 8-156020 A as a pertinent technique). That, however, would bring the display portions (colored layers) into contact with the molding resin at high temperature, resulting in the printed characters, etc. being washed away.
As a third solving method, it might be possible to use a resin film 9 of very high degree of transparency. Such a film, however, is likely to be rubbed and flawed when placed in the mold or during storage. Due to the high degree of transparency, the flaw would be so much the more conspicuous;
Then, this difficulty in obtaining clearness in the recognition of the display portions 9d due to the presence of the resin film 9 is experienced not only in the case of the film key sheet 4 having the two types of key portions 7 and 8 of different appearances but also in the case of a film key sheet having in a resin film exclusively a plurality of key portions of the same type as the key portions 8.